European patent EP 0 902 157 B1 discloses safeguarding doors with a light curtain which is mounted in the plane of motion of a door. So that the door is not detected as an object, monitoring beams of the light curtain are blocked with respect to object detection by, in each case, a monitoring beam lying behind in the direction of motion of the door, before a front edge of the door meets the respective monitoring beam. In this way, the monitoring beams of the light curtain are successively no longer available for object detection during a closing motion of the door.
However, in such a configuration it is conceivable that an object which is to be detected is incorrectly interpreted as an approaching door and could therefore bring about undesired blocking of monitoring beams.